1. Field of the Invention
The present invention relates generally to iron Fischer-Tropsch catalysts. More particularly, the present invention relates to a method for preparing stable Fe(II)/Fe(III) nitrate and/or ferrous nitrate solutions to produce Fischer-Tropsch synthesis catalyst, and the catalyst produced thereby. Still more specifically, the present invention relates to a method of producing a Fischer-Tropsch catalyst by precipitating iron from an acid solution comprising a desired ratio of ferrous iron to ferric iron and wherein the acid solution is stable for a desired time period.
2. Background of the Invention
The Fischer-Tropsch (FT) technology is used to convert a mixture of hydrogen and carbon monoxide (synthesis gas or syngas) to valuable hydrocarbon products. Often, the process utilizes a slurry bubble column reactor (SBCR). The technology of converting synthesis gas originating from natural gas into valuable primarily liquid hydrocarbon products is referred to as Gas-To-Liquids (GTL) technology. When coal is the raw material for the syngas, the technology is commonly referred to as Coal-To-Liquids (CTL). The FT technology is one of several conversion techniques included in the broader GTL/CTL technology.
One of the primary difficulties encountered in using iron-based catalysts for carrying out the FT reaction in a slurry bubble column reactor (SBCR) is the breakdown of the initial catalyst particles into very small particles, i.e. less than 5 microns in size. Although the small particle size is advantageous for increasing surface area and reaction rate of the catalyst, the problem lies in separating the small catalyst particles from the wax slurry medium. Separating the catalyst particles from the wax is desired since the iron catalyst when operated under the most profitable conditions wherein wax is produced utilizes removal of the wax from the reactor to maintain a constant height of slurry in the reactor.
It is impossible to determine the actual attrition resistance that is sufficient without knowing the type of reactor system, the type of wax/catalyst separation system and the system operating conditions.
Heretofore, attempts at developing strengthened iron-based catalysts have focused on producing the strongest possible catalysts, regardless of the actual strength sufficient for a particular system. Such approaches sacrifice activity and selectivity for catalyst strength which may exceed that which is sufficient. Most of the prior art has focused on attempting to maximize strength of the catalyst without due regard for the negative impact of high levels of strengthener, e.g. silica, on activity and selectivity. Further, tests for catalyst strength have been carried out ex-situ, i.e. outside the SBCRs. Many of the tests have been conducted in a stirred tank reactor (autoclave) which subjects the catalyst to severe shearing forces not typically encountered in slurry bubble column reactors.
Improved catalyst strength can be achieved by depositing the iron on a refractory support such as silica, alumina or magnesia or by adding a structural promoter to the baseline catalyst. The challenge is to strengthen the catalyst without appreciably compromising the activity and selectivity of the catalyst.
Formation of strengthened FT iron catalysts which utilizes a desired ratio of ferric to ferrous iron in an acid solution from which precipitation of iron phase occurs has been reported in U.S. patent application Ser. No. 12/198,459 filed Aug. 26, 2008 and entitled, “Strengthened Iron Catalyst for Slurry Reactors.” The disclosure of U.S. patent application Ser. No. 12/198,459 is incorporated hereby herein for all purposes not inconsistent with this disclosure. Ferrous nitrate solution is reported to be very unstable in the literature. Production of stable ferrous nitrate solutions and stable solutions comprising a desired ratio of ferrous to ferric iron is challenging, and the production thereof will lead to more consistent iron-catalyst formation and a decrease in the time and costs of catalyst formation.
Accordingly, there is a need for a method of producing stable ferrous nitrate solutions and stable nitrate solutions comprising a desired ratio of ferric to ferrous iron. Also needed is a method of producing an iron FT catalyst which incorporates the use of the stable iron nitrate solutions. Use of these methods should desirably allow production of an iron FT catalyst which has resistance against breakdown during FT reaction and also maintains high activity and selectivity toward high molecular weight hydrocarbons. A method for stabilization of Fe(II)/Fe(III) nitrate solution should enhance the rate of the catalyst manufacturing process at dissolution and/or precipitation steps, desirably increase reproducibility of catalyst manufacture.